Please refer to FIG. 1 and FIG. 2, which show a conventional axial-flux permanent-magnet motor. It is known that a conventional axial-flux permanent-magnet motor, as the one shown in FIG. 1 and FIG. 2 that is primarily comprised of a stator 1 with a coil 11 and a rotor 2 with a permanent magnet 21, can be tuned and adapted to perform at a high-speed/low-torque mode or at a low-speed/high-torque mode simply by changing its air gap width, which can be achieved through the activation of its internal mechanism. That is, when the air gap is widened, the air-gap magnetic flux density will decrease and thus the rotation speed of the motor is increased, but on the other hand, when the air gap is narrowed, the torque of the motor is increased. Moreover, the rated speed of the motor can also be increased effectively by the use of a field-weakening control method or a phase advance method, using that the current phase of the stator 1 is controlled for suppressing the magnetic flux by counter magnetic flux and thus the rated speed of the motor can be increased effectively. However, for enabling a conventional motor to use an internal mechanism to adjust the air gap width against the normal force from the rotor 2, the structure of the motor can be very complex; and if the field-weakening control method or the phase advance method is used, in a long run that the permanent magnet 21 will be demagnetized and consequently the motor will be damaged.
There are already many studies for overcoming the aforesaid shortcomings. One of which is an ultra-thin motor, disclosed in TW Pat. No. I303122, by that not only the assembly and mass production efficiencies of the motor are improved, but also the problem troubling the thinning of conventional radial air-gap motors, such as the stacking of magnetic materials, is resolved.
The second such study relates to the internal mechanism design inside motors, disclosed in TW Pat. No. I253800, which is substantially a brushless direct current (DC) motor with planetary gears capable of switching the engagement between different gears for changing gear ratio and thus causing the output torque of the motor to increase or decrease accordingly.
The third such study relates to a motor that is disclosed in TW Pat. No. I306324. The motor is configured with a controller to be used for controlling its planetary gears to adjust the air gap width of the motor, and consequently, changing the torque of the motor.
The fourth such study relates to a motor control method designed for changing the output characteristics of a variable-winding brushless motor, disclosed in TW Pat. No. 401923, which focuses on the power drive control of an electric scooter, so as to achieve optimal control efficiency and the required torque at different road speeds (high starting torque at low speed and with a good range of speeds). The approach used is based on changing the winding connection at motor's stator, so that there are two sets of windings on the variable-winding brushless motor, whereas a closed-loop torque control scheme is proposed to integrate different characteristics by combining the changing winding connection structure and torque-speed control.
The fifth such study relates to a method for adjusting phase change opportunity in brushless DC motor basing upon its rotation speed, disclosed in TW Pat. No. 483231, in which current phase of the motor is adjusted for affecting the motor in a manner similar to the conventional field-weakening control method or the phase advance method, so as to increase the rated speed of the motor without having to change the structure of the motor or its relating driving apparatuses.
The sixth such study relates to a hub motor mechanism, disclosed in U.S. Pat. No. 6,974,399 B2, in which a one-way clutch is connected between a cover of the hub motor and the planetary gear system for gear change as the planetary gear system is received inside the hub motor. Thereby, not only the speed and torque variations due to the varying gear ratio controlled by the planetary gear system can be achieved, but also the transmitting efficiency of the hub motor can be improved by the use of the planetary gear system and thus the gear change of the hub motor can be performed smoothly.
The seventh such study relates to an automatic air gap varying means for modulating the output characteristic of a motor, disclosed in U.S. Pat. No. 6,404,097 B1. In operation, as the speed of rotation speed of the motor increases, a centrifugal force is generated which acts to bend the air gap varying means, and thereby, increasing the size of the air gap; and vice versa that a subsequent decrease in rotation speed will reduce the magnitude of the centrifugal force generated and allows the air gap varying means resume its original position so that the size of the air gap is decreased causing the output torque of the motor to increase.
The eighth such study relates to a rotating electric machine whose output characteristics can be easily and freely adjusted and varied even in operation, disclosed in U.S. Pat. No. 7,468,568 B2. According to this configuration, as the electric rotating machine is housed in a housing with the rotating shaft as an axle shaft, and the adjustment motor is positioned with its output shaft lying in the vehicle front-to-rear direction, the housing can be made slim. That is to say, the electric rotating machine can be used as an in-hub type power unit, and when used as an in-wheel motor, for example, a slim and compact power unit can be implemented. Moreover, it is able to adjust the gap between the rotor and stator by the internal gear linkage of the rotating electric machine even when the rotor is rotating.
Nevertheless, with reference to all the aforesaid patents, the currently available disc motors for electronic devices are all small power motor since the coils capable of being embedded therein are micro/nano scaled that they can not sustain the current loads of other common-sized motors, not to mention that there is no back iron constructed on the stator of those currently available disc motors, the torques that they can provided are limited. Moreover, for the speed change mechanisms that are used currently in those conventional motors, in addition to the shortcomings of highly complicated structure and high maintenance cost resulting from the regular maintenance requirements upon the internal gear box and transmission when the speed changing is enable through the internal mechanism of the motors, when the speed changing of the motor is achieved by adjusting current phase of the motor for affecting the motor in a manner similar to the conventional field-weakening control method or the phase advance method, it is disadvantageous in that the permanent magnet can be demagnetized in a long run and consequently the motor will be damaged.
Therefore, it is in need of an adjustable axial-flux disc motor capable of modulating its magnetic flux intensity without the help from any addition transmission mechanism, and consequently changing the air-gap flux density for enabling the rotation speed of the motor to vary according while preventing the motor from being damaged by the demagnetization of its magnets.